Photo of David A. Williams

David A. Williams

Adjunct Professor of Physics
Ph.D. Harvard University, 1987

Office: Room 319, Natural Science 2
Phone: (831) 459-3032
FAX: (831) 459-5777

Research Interests

I am a member of the VERITAS and CTA collaborations, pursuing the study of high-energy gamma-rays from astrophysical objects. These experiments adapt experimental techniques developed for particle physics to explore outstanding problems in high-energy astrophysics and gamma-ray astronomy. I am particularly interested in understanding the nature of gamma-ray bursts and the high-energy emission mechanism of active galactic nuclei. I participate in both the development of the instrumentation for the experiments, especially the electronics and photodetectors, and in the analysis and interpretation of the data.

VERITAS is one of the premier instruments worldwide for observations of gamma rays above ~100 GeV. During the summer of 2012, we helped complete an upgrade of the cameras with higher sensitivity photomultiplier tubes. We have an active program of source observations and data analysis.

CTA is a proposed concept for a future gamma-ray instrument with about ten times better sensitivity than VERITAS. Our group is evaluating photosensors for the camera of a novel telescope Schwarzschild-Couder design using a secondary mirror and a compact camera. I am currently the chair of the CTA-US group.

I am also an Affiliated Scientist with the Fermi Large Area Telescope (LAT), with a particular interest in studies combining high-energy gamma-ray data from the LAT with very high-energy gamma-ray data from VERITAS.

Collaboration Involvement: VERITAS CTA Fermi-LAT

Selected Recent Publications

Gamma-Rays from the Quasar PKS 1441+25: Story of an Escape, A. U. Abeysekara et al., Astrophys. J. Lett. 815, L22 (2015).

The Extragalactic Background Light, the Hubble Constant, and Anomalies: Conclusions from 20 Years of TeV Gamma-Ray Observations, J. Biteau and D. A. Williams, Astrophys. J. 812, 60 (2015).

Constraints on Very High Energy Emission from GRB 130427A E. Aliu et al., Astrophys. J. Lett. 795, L3 (2014).

The Blazar Emission Environment: Insight from Soft X-Ray Absorption, A. Furniss et al., Astrophys. J. 770, 109 (2013).

The Firm Redshift Lower Limit of the Most Distant TeV-detected Blazar PKS 1424+240, A. Furniss et al., Astrophys. J. Lett. 768, L93 (2013).

IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array, R. C. Gilmore, A. Bouvier, V. Connaughton, A. Goldstein, N. Otte, J. R. Primack, and D. A. Williams, Experimental Astron. 35, 413 (2013).

Detection of Pulsed Gamma Rays Above 100 GeV from the Crab Pulsar, E. Aliu et al., Science 334, 69 (2011).

Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy, M. Actis et al., Experimental Astron. 32, 193 (2011).

A connection between star formation activity and cosmic rays in the starburst galaxy M82, V. Acciari et al., Nature 462, 770 (2009).

Radio Imaging of the Very-High-Energy γ-Ray Emission Region in the Central Engine of a Radio Galaxy, V. A. Acciari et al., Science, 325, 444 (2009).

Milagro Observations of Multi-TeV Emission from Galactic Sources in the Fermi Bright Source List, A. A. Abdo et al., Astrophys. J. Lett. 700, L127 (2009).

Discovery of Localized Regions of Excess 10-TeV Cosmic Rays, A. A. Abdo et al., Phys. Rev. Lett. 101, 221101 (2008).

Complete list of VERITAS publications

More about Research

Particles more energetic than any man-made accelerator can produce are continuously bombarding the earth. We do not yet understand where these high-energy cosmic rays come from or how they are accelerated to such tremendous energies in astrophysical systems. The vast majority are charged particles which are deflected in the galactic magnetic fields, such that their arrival direction tells little about whence they came. Gamma rays, while much rarer, arrive at earth unperturbed, still appearing to come from their true source. By finding and studying sources of high-energy gamma rays, we can identify objects where particle acceleration is taking place and learn about the mechanisms involved.

One important class of gamma-rays sources is blazars. They are galaxies with an active nucleus (an AGN, for "active galactic nucleus") containing a supermassive black hole (as much as a billion times the mass of the Sun) at the center which is accreting material and converting some of the gravitational energy into jets of relativistic particles ejected along the rotation axis. For blazars, we are looking right down the barrel of one of these jets which is pointed along the line of sight to Earth. Learning how these jets are produced and what processes are taking place in them is one of the main objects of gamma-ray research.

Satellites detect short bursts of low-energy gamma rays about once a day lasting from a fraction of a second to a few tens of seconds. Each gamma-ray burst (GRB) is from a different and unpredictable location, making them challenging to study. While our knowledge of GRBs has been rapidly improving, it is far from complete. By searching for very-high-energy gammas (>100 GeV) from bursts, we can test models of the gamma-ray emission.

Low energy photons from the 2.7 K cosmic microwave background or infrared starlight can collide with high-energy gamma rays to produce an electron-positron pair, effectively absorbing some of the gamma rays. For extragalactic sources, the amount of this absorption can probe the amount of intergalactic IR radiation. UCSC theorist Joel Primack and collaborators have been leaders in predicting how measurements of this absorption could distinguish between different cosmological models, and our group has a major role in using gamma-ray data to test these models.

VERITAS is a powerful system of four 12-m imaging atmospheric Cherenkov Telescopes completed in winter 2007 and upgraded in 2009, 2011 and 2012. The telescopes collect Cherenkov light produced in the atmosphere by relativistic particles in gamma-ray showers.

Other Interests

When not thinking about physics, I enjoy playing violin in the Peninsula Symphony, hiking, and biking.

Page revised March 2016